1. Field of the Invention
This invention relates to a system and method for forming holograms and, more particularly, to a system and method for forming holograms on a production basis in an efficient and economic manner while substantially reducing undesirable spurious hologram noise recordings.
2. Description of Related Art
High quality holographic optical elements are used in diffraction optics display systems, such as Heads Up Displays (HUD), for advanced aircraft, helmet mounted displays, laser eye protective devices, narrow band reflective filters, and holographic high gain screens for simulators. These are only a few of the many uses of high quality reflective holograms. There have existed problems in the prior art in providing economical mass production of reflective holographic optical elements where production units are "copies" of either a master reference object or a master hologram, which provides an aspheric reflective wavefront for a HUD.
A constant problem in diffraction optic display systems utilizing a hologram has been a degradation of the holographic images as a result of the effects of spurious reflection and transmission hologram recordings that are frequently generated during the holographic replication process. Some of the most objectionable of those spurious noise holograms have been found to be generated by reflections from surfaces which are interfaces of materials of different indexes of refraction, such as air/glass interfaces of the transparent surfaces of the recording cover plate, the substrate, the recording medium, and optical elements. These reflections can combine with the primary holographic beams at the recording film to form both spurious reflection hologram recordings and spurious transmission hologram recordings. As a result, a subsequent display system will create ghost images from the spurious reflection hologram recordings and rainbow-like flare patterns from the spurious transmission hologram recordings.
The prior art has attempted to address these problems in numerous different ways. One approach has been to minimize the differences in index refraction by attempting to match the indexes of refraction with an index matching fluid, such as a mineral oil. Attempts have been made to immerse a recording module in an index of refraction matching oil bath. Another approach has been to form a hologram with energy beams impinging the recording film supporting elements at Brewster's angle.
U.S. Pat. Nos. 4,458,977, 4,458,978, and 4,456,328 disclose prior art approaches to eliminating the noise caused by a glass/air interface of an outer surface cover plate by moving the cover plate to change the phase of the reflected rays relative to the primary beams during the recording period so that spurious holograms are not formed. The rate of motion or phase change in accordance with these solutions is a function of the exposure time, which itself is a function of the sensitivity of the recording medium. The total amount of the motion is designed to require a phase change of one or more half wavelengths in the reflected noise beams to nullify any constructive or destructive interference patterns. These approaches have been proposed to solve the complex problems involved in the manufacturing of reflective holographic optical elements for use in heads up displays.
Generally, in providing these solutions in the prior art, there is a layer of index matching fluid, such as an appropriate mineral oil, which will vary in thickness during the cover movement. A relatively thick image degrading layer of index matching fluid has the capacity to degrade the surface of the reference object, such as an aspheric mirror, creating moving striations. These moving striations cause fringe degradation and frequently require the oil to be cleaned. Initially, a double beam system has been utilized, which required days of stabilization before an appropriate exposure. Subsequently, a master aspheric mirror single beam system was utilized; however, it still required many hours of stabilization and the use of relatively skilled labor.
Additionally, in the prior art approaches, only the outer surface, that is, the glass/air interface elements, could be provided with an anti-reflective coating. If an inner surface was required to be coated for optimum use in air, the anti-reflective coating would have to be added at a later time after the exposure, such as by adding an anti-reflective coated glass, which would add further weight, or by depositing a standard anti-reflective coating, which would frequently thermally destroy the hologram, or by depositing a cold anti-reflective coating, which would be less efficient and more fragile. Finally, this example of prior art required a piezoelectrically controlled exposure cover that had to be appropriately mounted and calibrated prior to exposure, and also taken into account in the design of the optical system.
U.S. Pat. No. 4,478,490 discloses an alternative method of reducing coherent noise content through the modulation of the position of an apodizer in the optical path during an exposure. The apodizer permits the amplitude of the wave front to be modified to alter a point source response, that is, to change in a predetermined way the point spread function whereby the fringe patterns created by the apodizer are unstable and hence reduce the noise content of the transmitted radiation.
Another prior art attempt to remove noise employs the use of a laser source without an etalon to reduce noise holograms from a surface further away than approximately two inches (a typical coherence length for a large argon laser). While this approach can reduce noise, it has the disadvantage of being a step process with either the etalon being in and the coherence length being many meters, or the etalon being not in, and the coherence length being on the order of inches. This relatively new approach can be acceptable where a distance of approximately one-quarter-inch is required, such as in a HUD-type hologram with an aspheric mirror surface.
The prior art has frequently recognized the desirability of reproducing copies from a master hologram. A theory of such copying of holograms is set forth in Brumm, "Copying Holograms," Applied Optics, Volume 5, No. 12, page 1946, December 1966. Reference is also made to U.S. Pat. No. 3,758,186, U.S. Pat. No. 3,639,031, U.S. Pat. No. 3,647,289, U.S. Pat. No. 4,312,559, and U.S. Pat. No. 4,530,564 to disclose other methods of copying holograms.
The prior art is still seeking an optimum method and apparatus for the reproduction of multiple hologram optical elements in an economical and efficient manner, including improving the format of providing a recording module for HUD manufacturing and reducing the creation of noise in the HUD hologram.